Heteroantisera prepared to homogenates of murine brain recognize multipotential hemopoietic stem cells in addition to various other cell types. Published reports and preliminary experiments suggest that an antigen or antigens present on stem cells and also present in brain differs at least quantitatively and probably qualitatively from brain borne antigens common to the other cell types. Preliminary experiments indicate that the majority, but not all stem cells, display this antigen(s). It is proposed to utilize this antigen(s) to develop direct assays for the detection and enumeration of stem cells in mice and in humans. Antisera prepared against whole brain homogenates and then extensively absorbed with various tissues are insufficiently specific for this purpose so the stem cell antigen(s) will be isolated from brain, characterized, and specific antibodies prepared. These antibodies will be extensively tested for specificity, fluorochrome or radiolabeled and used in the study of stem cell distribution, proliferation, and differentiation in the mouse, and hopefully will be useful for monitoring stem cells in human health and disease states.